eddy wrote:

Surely this would be a good way to destroy a plant that it proving
hard to get rid of.

It's a lousy choice. Caustic soda is an aggressive corrosive chemical
that will take your flesh off.

Sodium chlorate will work a lot better against plants if you don't mind
its persistence.

Cheap industrial ways to make caustic soda have a tendency to cause
mercury pollution. Kitchen sink chemicals are not necessarily enviro
friendly. Glyphosate would probably be less environmentally damaging all
round.

Maybe salt ,sodium chloride is another way instead of using chemical
weed killers.
Would regular doses of salt water kill off a plant ?

If you use enough then yes. But don't expect to grow much there for a
long while afterwards.
(much more effective and persistent in dry countries)

Regards,
Martin Brown